1. Field of the Invention
The present invention relates to a fuel cell system and a warming up completion determining method for the fuel cell system.
2. Discussion of the Background
In the field of fuel cells, a fuel cell stack is known. The fuel cell stack includes a plurality of unit fuel cells (hereinafter referred to as “unit cells”) that are stacked on top of one another. Each of the unit cells is plate shaped and includes a membrane electrode assembly (MEA) that is formed by sandwiching a solid polymer electrolyte membrane between an anode and a cathode and a pair of separators placed on both sides of the MEA. In each unit cell of the fuel cell stack, hydrogen gas serving as fuel gas (anode gas) is supplied to a fuel gas channel between the anode and the separator on the anode side, and air serving as oxidant gas (cathode gas) is supplied to an oxidant gas channel between the cathode and the separator on the cathode side. Accordingly, hydrogen ions produced by a catalytic reaction in the anode pass through the solid polymer electrolyte membrane, move to the cathode, and cause an electrochemical reaction with oxygen in the air in the cathode, so that electric power is generated. In accordance with the generation of electric power, water is generated in the oxidant gas channel (hereinafter referred to as “generated water”).
Some fuel cell stacks of this type are provided with an informing unit configured to inform a driver of whether the power generation performance of the fuel cell stack has reached a predetermined range after warming up has been performed after activation of the fuel cell stack has started in order to realize stable generation of electric power. As such a configuration, the following configuration is known. That is, the time from when a system starting instruction is received until when the informing unit is operated is changed in accordance with an elapsed time from when a system stopping instruction was previously received until when the system starting instruction is received (e.g., see Japanese Unexamined Patent Application Publication No. 2008-218165).
In the above-described fuel cell stack, the power generation performance thereof may not be ensured even if the temperature in the fuel cell system is not low during activation. Specifically, in the case where the previous activation was performed under a low-temperature environment and an activation time was short (hereinafter referred to as “low-temperature short-time operation”) and where the temperature in the fuel cell system is not low at the next activation, generated water generated during the previous activation may remain in a frozen state in the fuel cell stack. That is, generated water freezes if the fuel cell stack is activated under a low-temperature environment. The freezing is overcome by continuing a warming up operation, but is not overcome if the warming up operation is performed for a short time, and the generated water remains as ice in the fuel cell stack. In this case, the ice covers the individual cells, reducing effective areas for generating electric power of the individual cells. This causes a problem that the power generation performance of the fuel cell stack is not ensured until the freezing is overcome. The temperature for ensuring the power generation performance of the fuel cell stack is typically about 70 to 80° C.
In the above-mentioned example of the related art, a decrease in concentration of hydrogen gas is dealt with as a problem, but freezing of generated water due to a low-temperature short-time operation is not considered. That is, in the configuration according to the related art, the informing unit is operated on the basis of the time from the previous system stopping instruction to a system starting instruction regardless of an activation environment of the fuel cell stack. For this reason, the informing unit may be operated before freezing of generated water is overcome in the case of low-temperature activation or activation after a low-temperature short-time operation. As a result, a desired power generation performance is not obtained, or current for generating electric power or the amount of supplied air is increased in order to obtain an output equivalent to that in normal-temperature activation, which causes a driver to feel uncomfortable.